A communication apparatus such as a mobile phone or a wireless LAN is required to ensure the accuracy of a transmission signal, and also operate with a low power consumption. Such a communication apparatus uses a transmission circuit that is small in size, operates at a high efficiency, and outputs a transmission signal having a high linearity.
Examples of a conventional transmission circuit include a transmission circuit that generates a transmission signal using a modulation scheme such as quadrature modulation (hereinafter referred to as a “quadrature modulation circuit”). Further, a polar modulation circuit is known that is smaller than, and also operates at a higher efficiency than, the quadrature modulation circuit. Generally, polar modulation allows the use of a power amplifier, included in a transmission circuit, in a saturated state. This enables the power amplifier to output a desired power at a high efficiency. This makes it possible to reduce the power consumption of the transmission circuit as compared to the case where the transmission circuit operates by quadrature modulation.
In addition, polar modulation makes it possible to reduce the device size of the power amplifier as compared to the case of quadrature modulation. This makes it possible to set an increased efficiency of the power amplifier by switching the operation mode to that of quadrature modulation only at the time of a low output.
FIG. 9 is a diagram showing a conventional transmission circuit 900. In FIG. 9, the conventional transmission circuit 900 includes a radio frequency integrated circuit (hereinafter referred to as “RF-IC”) 910, an envelope management integrated circuit (hereinafter referred to as “EM-IC”) 920, and a power amplifier (PA) 930.
When the output of the conventional transmission circuit 900 is a high output, the conventional transmission circuit 900 operates in a polar modulation mode. In the polar modulation mode, a transmission signal input to the RF-IC 910 is separated into a phase component and an amplitude component. The amplitude component is converted into an amplitude-modulated signal, and the amplitude-modulated signal is input to the EM-IC 920. The EM-IC 920 generates a control voltage Vout on the basis of the input amplitude-modulated signal, and supplies the control voltage Vout to the power amplifier 930. The phase component is converted into a phase-modulated signal having a constant amplitude, and the phase-modulated signal is input to the power amplifier 930. The power amplifier 930 amplifies the power of the input phase-modulated signal on the basis of the control voltage Vout supplied from the EM-IC 920, and outputs the resulting phase-modulated signal as a transmission signal.
On the other hand, when the output of the conventional transmission circuit 900 is a low output, the conventional transmission circuit 900 operates in a quadrature modulation mode. In the quadrature modulation mode, a transmission signal input to the RF-IC 910 is converted into a quadrature-modulated signal on an input path side, and the quadrature-modulated signal is input to the power amplifier 930. The EM-IC 920, which is on a power supply path side, generates the control voltage Vout on the basis of a power supply voltage, and supplies the control voltage Vout to the power amplifier 930. The power amplifier 930 amplifies the power of the input quadrature-modulated signal on the basis of the control voltage Vout supplied from the EM-IC 920, and outputs the resulting quadrature-modulated signal as a transmission signal.
As described above, in accordance with the operation mode of the conventional transmission circuit 900, the EM-IC 920 switches the control voltage Vout, which is to be supplied to the power amplifier 930, to that for the polar modulation mode or that for the quadrature modulation mode. Further, Patent Literature 1 discloses a power supply device that achieves a high efficiency by switching between an LDO and a switching regulator.